This type of clamping apparatus is known from a variety of printed publications. For example, DE 199 24 469 A1 shows a welding bracket for workpieces, especially for the laser transmission welding of faying surfaces made of plastic, where the faying surfaces are found on the circumferential lip around the edges of the two-part workpiece. One workpiece is held by a carrier for the suspension of the component. The second component is applied with a clamping chuck, which is held outside of the actual welding line for the application of clamping pressure to the faying surface.
For welding devices as per DE 95 10 493 A1, a pressure effect is caused by hydraulic, pneumatic, rolling or other downholders. Explicitly a pressure roller is released, which runs along the radiated area next to the laser beam. A considerable disadvantage of this process is that by moving the pressure roll a tangential force is applied along the direction of movement of the roll. This force causes tensions in the join partner penetrated by radiation. Furthermore, a damage of the surface may be caused by the pressure roll attached to the workpiece surface.
U.S. Pat. No. 5,049,720 A shows a laser welding device for the lid of a plastic container, where the lid is pressed on the container lip in the area of the weld seam by a double-ring-shaped clamping fixture. The gap between both clamping rings is thereby completely covered by a laser-transparent window, so that the area underneath is sealed when the lid is fixed, and additionally subjected to overpressure. This method is meant to make the lid sit evenly on the container. The laser beam passes through the window and the gap between the two rings and hits the welding area. The clamping pressure is brought about by a central pressure cylinder situated above the workpiece. This configuration is not meant for smaller components in need of welding.
In terms of the invention's background, a quick look should be taken at the primary area of use, namely transmission welding. Here, the transfer of the necessary laser power to the welding area is achieved by the laser beam being run through the lid of one component, which is transparent or at least translucent for the laser beam's wave range. The laser energy is at least partly absorbed by the lower of the joining partners. The resulting heat causes the lower joining partner to partially melt; the first component is also partially melted through the heat transfer from the second component to the lid. A connection between the two joining partners results through the pressure on the welding area. The laser beam works its way along the weld seam.
To produce the pressure effect, DE 199 24 469 A1—which represents up-and-coming state-of-the-art technology—shows an outer clamping apparatus where the opposing clamping pressure on the joining partners is applied to one side of the weld seam. This leads to a one-sided strain on the radiated joining partner along the weld seam, which is a drawback for the welding quality. The pressure distribution diminishes toward the welded seam and can cause an outer deformation of the component. As such, strain can arise in plastic structures, which can cause cracks in or next to the weld seam after the melted portions have solidified.
Further, the outer clamping apparatus is hard to use when the components offer little or no material to which to attach it outside of the weld seam.